U.S. patent number 7,218,881 [Application Number 11/157,820] was granted by the patent office on 2007-05-15 for developing apparatus features first and second developing members and image forming apparatus having the same.
This patent grant is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Shigeru Tanaka.
United States Patent |
7,218,881 |
Tanaka |
May 15, 2007 |
**Please see images for:
( Certificate of Correction ) ** |
Developing apparatus features first and second developing members
and image forming apparatus having the same
Abstract
A developing apparatus provided with first and second members
disposed on an upstream and downstream sides, respectively, and
opposed to an image bearing member. When a closest distance between
the first developing member and the image bearing member is defined
as SD1 (m), and the closest distance between the second developing
member and the image bearing member is defined as SD2 (m), and the
peak-to-peak voltage of a developing bias applied to the first
developing member is defined as V1 (V), and the peak-to-peak
voltage of the developing bias applied to the second developing
member is defined as V2 (V), and the volume resistivity of the
carrier is 1.0.times.10.sup.x.OMEGA.cm,
V1/SD1-V2/SD2.ltoreq.(0.65x-4.5).times.10.sup.6(V/m),
1.2.times.10.sup.6.ltoreq.V1/SD1.ltoreq.(0.35x+3.5).times.10.sup.6(V/m),
(0.30x-1.5).times.10.sup.6.ltoreq.V2/SD2.ltoreq.9.0.times.10.sup.6(V/m),
and 9.0.ltoreq.X.ltoreq.13.0 are satisfied.
Inventors: |
Tanaka; Shigeru (Toride,
JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
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Family
ID: |
35541524 |
Appl.
No.: |
11/157,820 |
Filed: |
June 22, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060008300 A1 |
Jan 12, 2006 |
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Foreign Application Priority Data
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Jul 7, 2004 [JP] |
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2004-201171 |
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Current U.S.
Class: |
399/269;
399/53 |
Current CPC
Class: |
G03G
15/065 (20130101); G03G 2215/0609 (20130101); G03G
2215/0648 (20130101) |
Current International
Class: |
G03G
15/08 (20060101); G03G 15/09 (20060101) |
Field of
Search: |
;399/269,267,265,149,150,53 ;430/120,122 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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8-123196 |
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May 1996 |
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JP |
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11-161017 |
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Jun 1999 |
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JP |
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Primary Examiner: Chen; Sophia S.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
What is claimed is:
1. A developing apparatus comprising: a first developing member and
a second developing member disposed in succession from an upstream
side to a downstream side with respect to a rotation direction of
an image bearing member on which an electrostatic image is formed
and in opposed relationship with said image bearing member; and
applying means for applying a developing bias comprising a DC
voltage and an AC voltage superimposed one upon the other to said
first developing member and said second developing member, wherein
the developing bias is applied to said first developing member and
said second developing member and the electrostatic image is
developed with a developer including a toner and a carrier, and the
toner on the image bearing member is collected, and wherein when a
closest distance between said first developing member and the image
bearing member is defined as SD1 (m), and a closest distance
between said second developing member and the image bearing member
is defined as SD2 (m), and a peak-to-peak voltage of the developing
bias applied to said first developing member is defined as V1 (V),
and a peak-to-peak voltage of the developing bias applied to said
second developing member is defined as V2 (v), and a volume
resistivity of the carrier is defined as 1.0.times.10.sup.x
(.OMEGA.cm), V1/SD1-V2/SD2.ltoreq.(0.65x-4.5).times.10.sup.6(V/m),
1.2.times.10.sup.6.ltoreq.V1/SD1.ltoreq.(0.35x+3.5).times.10.sup.6(V/m),
(0.30x-1.5).times.10.sup.6.ltoreq.V2/SD2.ltoreq.9.0.times.10.sup.6(V/m),
and 9.0.ltoreq.X.ltoreq.13.0 are satisfied.
2. A developing apparatus according to claim 1, wherein when DC
component values of the developing bias applied to said first
developing member and said second developing member are the same,
and when a surface potential of a maximum density portion of the
electrostatic image is defined as Vlight (V), and a toner surface
layer potential on the image bearing member after the maximum
density portion of the electrostatic image has been developed by
said first developing member is defined as Vt1 (V), and a toner
surface layer potential on the image bearing member after the
maximum density portion has been developed by said second
developing member is defined as Vt2 (V),
0.15.ltoreq.(Vt2-Vt1)/(Vt2-Vlight).ltoreq.0.50 is satisfied.
3. A developing apparatus according to claim 1 or 2, wherein SD1,
SD2, V1 and V2 satisfy:
0.2.times.10.sup.-3(m).ltoreq.SD1.ltoreq.1.0.times.10.sup.-3 (m),
0.2.times.10.sup.-3(m).ltoreq.SD2.ltoreq.1.0.times.10.sup.-3 (m),
500(V)V1.ltoreq.3000 (V), and 500(V)V2.ltoreq.3000 (V).
4. A developing apparatus according to claim 1 or 2, wherein an AC
component waveform of the developing bias applied to said first
developing member and said second developing member is a waveform
in which a rectangular portion and a discontinued portion are
alternately repeated.
5. A developing apparatus according to claim 1 or 2, wherein the
same developing biases are applied to said first developing member
and said second developing member by the said applying means.
6. An image forming apparatus comprising: an image bearing member
on which an electrostatic image is formed; a developing apparatus
for toner-developing the electrostatic image; and transferring
means for transferring a toner-developed image to a sheet, said
developing apparatus being a developing apparatus according to
claim 1 or 2.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a developing apparatus for developing a
photosensitive member and having a cleaning function of removing a
toner residual on the photosensitive member, and an image forming
apparatus provided with this developing apparatus.
2. Description of Related Art
In image forming apparatuses such as a copying machine, a printer
and a facsimile apparatus adopting an electrophotographic process,
and a compound machine of these there is a case where in order to
collect any untransferred toner not transferred to a transfer
medium but residual on a photosensitive member, the untransferred
toner is collected by a so-called "cleanerless method" of
collecting the untransferred toner by a developing device without
using a cleaner apparatus contacting with the photosensitive
member.
When this cleanerless method is used, the untransferred toner can
be recycled without being disused. Also, a frictional member does
not contact with the photosensitive member and therefore, the
abrasion of the photosensitive member can be prevented.
Accordingly, a running cost can be reduced.
Particularly, when a two-component developing method using a
developer consisting of a mixture of a toner and a magnetic carrier
is used as a developing method, collectability of the untransferred
toner by a magnetic brush and the re-chargeability of the collected
toner are improved. Therefore, the two-component developing method
is suited for the cleanerless method.
The following developing apparatus is known as an example of a
developing apparatus adopting the cleanerless method.
That is, a cleaning and developing apparatus having at least two
rollers disposed in opposed relationship with a photosensitive
member, and using a two-component developer, wherein a roller
disposed upstream with respect to the rotation direction of the
photosensitive member is a cleaning roller for cleaning the
photosensitive member, and a roller disposed downstream of the
cleaning roller with respect to the rotation direction of the
photosensitive member is a developing roller for developing an
electrostatic latent image on the photosensitive member with the
two-component developer (see Claim 1 and FIG. 3 of Japanese Patent
Application Laid-open No. H08-123196).
This developing apparatus has disposed therein the roller given a
cleaning function and the roller given a developing function, to
thereby make the collectability of the untransferred toner and the
developing property which are important in the cleanerless method
compatible.
The developing apparatus, as shown in FIG. 7 of the accompanying
drawings, makes a cleaning roller 11 and a developing roller 12
carry a two-component developer thereon, and applies predetermined
biases (DCVB1+AC) and (DCVB2, respectively, thereto and therefore,
is of a construction of a so-called "multi-stage developing type"
which develops a latent image on a photosensitive drum 1 by the use
of virtually two developing rollers.
However, the heretofore proposed cleanerless methods including the
above-described conventional example have suffers from the
following problems.
When a formed image includes many portions of high density, the
amount of toner consumed by a developing device becomes great.
Therefore, usually in the two-component developing method, a toner
corresponding to the consumed amount is adapted to be sequentially
supplied and thus, the amount of toner supply has also been
increased. In such a case, the imparting of charges to the toner by
the agitation of the two-component developer is liable to become
insufficient, and in some cases, the fogging characteristic itself
of the developing device has been somewhat aggravated. The
aggravation of the fogging characteristic increases a fogged toner
on the photosensitive member and also induces a reduction in
cleaning performance.
Also, in the case of the cleanerless method having no cleaner
member contacting with the photosensitive member, an increase in
the toner not removed and the fogged toner has in some cases led to
the vicious circle that the hindrance of charging is caused during
the charging of the photosensitive member to thereby cause a
further increase in fogging. Particularly, when image formation
including many portions of high density has been continuously
effected, such a vicious circle has often been caused.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a developing
apparatus designed so as not to be reduced in cleaning performance
even if the amount of consumed toner becomes great.
It is also an object of the present invention to provide an image
forming apparatus which is provided with a developing apparatus
designed to be not reduced in cleaning performance even if the
amount of consumed toner becomes great, and which can obtain a good
quality of image.
In order to achieve the above object, the developing apparatus of
the present invention is provided with a first developing member
and a second developing member disposed in succession from an
upstream side to a downstream side with respect to the rotation
direction of an image bearing member on which an electrostatic
latent image is formed and in opposed relationship with the image
bearing member, and applying means for applying a developing bias
comprising a DC voltage and an AC voltage superimposed one upon the
other to the first developing member and the second developing
member, and designed such that the developing bias is applied to
the first developing member and the second developing member and
the electrostatic latent image is developed with a developer
provided with a toner and a carrier, and the toner on the image
bearing member is collected, wherein when the closest distance
between the first developing member and the image bearing member is
defined as SD1 (m), and the closest distance between the second
developing member and the image bearing member is defined as SD2
(m), and the peak-to-peak voltage of the developing bias applied to
the first developing member is defined as V1 (V), and the
peak-to-peak voltage of the developing bias applied to the second
developing member is defined as V2 (V), and the volume resistivity
of the carrier is 1.0.times.10.sup.X (.OMEGA.cm),
V1/SD1-V2/SD2.ltoreq.(0.65x-4.5).times.10.sup.6(V/m),
1.2.times.10.sup.6.ltoreq.V1/SD1.ltoreq.(0.35x+3.5).times.10.sup.6(V/m),
(0.30x-1.5).times.10.sup.6.ltoreq.V2/SD2.ltoreq.9.0.times.10.sup.6(V/m),
and 9.0.ltoreq.X.ltoreq.13.0 are satisfied.
In order to achieve the above object, the image forming apparatus
of the present invention is provided with an image bearing member
on which an electrostatic latent image is formed, a developing
apparatus for toner-developing the electrostatic latent image, and
transferring means for transferring the toner-developed image to a
sheet, wherein the developing apparatus is the above-described
developing apparatus.
These and other objects, features and advantages of the present
invention will becomes more apparent upon consideration of the
following description of the preferred embodiments of the present
invention taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a copying machine as an image forming
apparatus according to an embodiment of the present invention, and
a developing device as a developing apparatus according to a first
embodiment.
FIG. 2 is a schematic view of a carrier resistance measuring
apparatus.
FIG. 3 illustrates the relations among the latent image potential
of a photosensitive drum, the toner surface layer potential of the
developing device and the DC component of a developing bias.
FIG. 4 shows the relations among the latent image surface potential
on the photosensitive drum when the resistance value as a carrier
is 1.0.times.10.sup.7.OMEGA.cm, the DC component Vdev of the
developing bias, and toner surface layer potential Vt1 and Vt2
after the completion of development on a first developing roller
and a second developing roller.
FIG. 5 shows the relative relations between AC electric fields
applied to among the first developing roller, the second developing
roller and the photosensitive drum.
FIG. 6 shows the blank pulse waveform of the developing bias of a
developing device according to a second embodiment.
FIG. 7 is a schematic view of a conventional developing
apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A description will hereinafter be made of a copying machine as an
image forming apparatus and a developing device as a developing
apparatus according to an embodiment of the present invention.
Numerical values taken up in the present embodiment are reference
numerical values and the present invention is not restricted to
these numerical values. Also, the numerical values are approximate
numerical values and are not restricted to them.
The copying machine 50 as the image forming apparatus is provided
with a primary charger 21, a laser beam scanner 22, a
photosensitive drum 28, an auxiliary charging brush 32, and the
developing device 1 as the developing apparatus, a transfer charger
23 and a fixing device 25. The term "image forming apparatus"
covers a copying machine, a printer, a facsimile apparatus and a
compound machine of these, and is not restricted to the copying
machine. Also, the copying machine according to the present
embodiment will be described as an example of a monochromatic
copying machine, but may be a polychromatic copying machine which
is provided with a plurality of developing devices 1 and forms an
image of plural colors on transfer paper as a sheet.
Referring to FIG. 1, the photosensitive drum 28 is adapted to be
rotated (rightwardly rotated) at a predetermined speed in a
direction indicated by the arrow in FIG. 1 by a driving apparatus
(not shown). The photosensitive drum 28 is of a cylindrical shape
having a diameter of 60 mm. The peripheral speed of the outer
peripheral surface of the photosensitive drum 28 should preferably
be within a range of 50 mm/sec. to 500 mm/sec. In the present
embodiment, it is 200 mm/sec. The primary charger 21 having an
elastic layer on the surface layer thereof is in contact with the
photosensitive drum 28. A primary charging bias is applied to the
primary charger 21 by a charging bias voltage source (not shown).
The primary charging bias is a bias comprising a DC component -350
to -800 V and a sine wave of a peak-to-peak voltage 1200 to 1800 V
as an AC component superimposed one upon the other. The values of
the DC component and the AC component are determined in accordance
with the ambient temperature, humidity, etc., but herein the value
of the DC component is defined as -500 V and the value of the AC
component is defined as 1500 V.
The photosensitive drum 28 uniformly charged by the primary charger
21 is subjected to exposure according to image information by the
laser beam scanner 22. Thereby, an electrostatic latent image
according to the image information is formed on the photosensitive
drum 28. A portion of the photosensitive drum which is not
subjected to the exposure by the laser beam scanner 22 corresponds
to a blank portion on the image, and the surface potential Vdark
thereof is -500 V which is substantially the same as the DC
component of the primary charging bias. Also, the portion subjected
to the exposure by the laser beam scanner corresponds to a maximum
density portion on the image, and the surface potential Vlight
thereof varies in accordance with an exposure amount and the
photosensitive characteristic of the photosensitive drum 28, but
for convenience of description, it is defined as -100 V here. A
medium density portion on the image is formed by time-sharing the
light emitting time of the laser beam scanner 22.
The electrostatic latent image formed in this manner is developed
by the developing device 1. The developing device 1 is provided
with a developer container 2 containing therein a two-component
developer comprising chiefly a toner and a magnetic carrier, a
first developing roller 3A as a first developing member and a
second developing roller 3B as a second developing member disposed
in the opening portion of the developer container 2 which is
adjacent to the photosensitive drum 28, in succession from the
upstream side with respect to the rotation direction of the
photosensitive drum, and a developing bias voltage source 30 as
applying means common to the first and second developing rollers 3A
and 3B.
The closest distance between the first developing roller 3A and the
photosensitive drum 28 and between the second developing roller 3B
and the photosensitive drum 28 is 400 .mu.m. Each of the first
developing roller 3A and the second developing roller 3B is formed
of a cylindrical nonmagnetic material, and is adapted to be
rightwardly rotated in the direction indicated by the arrow at a
predetermined speed by a driving apparatus (not shown). Also, the
diameter of the first developing roller 3A is 24.5 mm, and the
diameter of the second developing roller 3B is 20 mm.
In the present embodiment, the peripheral speed ratio of the second
developing roller 3B to the first developing roller 3A may
preferably be within a range of 0.5 to 2.0. Also, the peripheral
speed of each developing roller may preferably be within a range of
1.2 times to 4.0 times relative to the peripheral speed of the
outer peripheral surface of the photosensitive drum 28. In the
present embodiment, the peripheral speed of the outer peripheral
surface of each of the first developing roller 3A and the second
developing roller 3B is 300 mm/sec., and is 1.5 times relative to
the peripheral speed of the outer peripheral surface of the
photosensitive drum 28 (the peripheral speed of the photosensitive
drum 28 is 200 mm/sec.) Also, the peripheral speed ratio of the
second developing roller 3B to the first developing roller 3A is
1.0.
A first magnet 5A and a second magnet 5B disposed stationarity
relative to the developer container 2 are contained in the
interiors of the first developing roller 3A and the second
developing roller 3B, respectively. Such magnetic poles as shown in
FIG. 1 are disposed in the first magnet 5A and the second magnet
5B.
The two-component developer is scooped up from within the developer
container 2 onto the second developing roller 3B by the magnetic
force of the pole S1 of the second magnet 5B. A regulating blade 4
is provided near the pole S1 and in the developer container 2 with
a predetermined gap (300 .mu.m) kept from the surface of the second
developing roller 3B. The regulating blade 4 is adapted to regulate
the layer thickness of the two-component developer on the second
developing roller 3B to a desired amount. The amount of the
two-component developer on the second developing roller 3B after
regulated is 40 mg/cm.sup.2. The layer-thickness-regulated
developer is conveyed by the rotation of the second developing
roller 3B and the magnetic conveying force of the second magnet 5B,
and contacts with the photosensitive drum 28 in a second opposed
portion 29B to the photosensitive drum 28. The two-component
developer passed through the second opposed portion 29B is further
conveyed and is delivered from the pole S2 of the second magnet 5B
to the pole N2 of the first magnet 5A. At this time, a low magnetic
flux density area in which the magnetic flux density on the surface
of the second developing roller 3B is approximate to 0 mT is formed
in the right portion of the second magnet 5B which is between the
pole S2 to the pole S1 and therefore, it never happens that the
two-component developer is conveyed to that portion. That is, the
two-component developer on the second developing roller 3B is all
delivered to the first developing roller 3A in the pole S2 to pole
N2 portion.
The two-component developer delivered to the first developing
roller 3A at the pole N2 is conveyed by the rotation of the first
developing roller 3A and the magnetic conveying force of the first
magnet 5A, and contacts with the photosensitive drum 28 in a first
opposed portion 29A to the photosensitive drum 28. At this time,
the amount of the two-component developer on the second developing
roller 3B is 40 mg/cm.sup.2 and the peripheral speeds of the outer
peripheral surfaces of the first developing roller 3A and the
second developing roller 3B are the same, namely, 300 mm/sec. and
therefore, the amount of the two-component developer on the first
developing roller 3A is 40 mg/cm.sup.2, the same as that on the
second developing roller 3B. The two-component developer passed
through the first opposed portion 29A is further conveyed and
arrives at a pole N4. Between the pole N4 to the pole N2, there is
formed a low magnetic flux density area similar to that between the
pole S2 to the pole S1, and the two-component developer is stripped
off from the first developing roller 3A and is collected into the
developer container 2.
As described above, the two-component developer is conveyed from
below to above in FIG. 1. In contrast, the surface of the
photosensitive drum 28 is downwardly moved in an area opposed to
the developing device 1. Thus, the directions of movement of the
electrostatic latent image and the two-component developer are
opposite to each other.
A developing bias is applied from a common developing bias voltage
source 30 to the first developing roller 3A and the second
developing roller 3B. The developing bias comprises a DC voltage
component and an AC voltage component superimposed thereon. The
value Vdev of the DC component is -350 V. Also, the AC voltage
component is a rectangular wave having a peak-to-peak voltage of
2000 V and a frequency of 6 kHz.
The potential difference between the potential Vdark of the
electrostatic latent image and the developing bias Vdev is a
fogging ensuring potential and is 150 V in the present
embodiment.
Description will now be made of the toner and the carrier
constituting the two-component developer.
The toner used in the present embodiment is a spherical toner
(polymerization toner) manufactured by a suspension polymerization
method of effecting the suspension polymerization of a monomer
composition comprising a styrene acryl monomer having a colorant
and a charge control agent added thereto in a water medium, and the
volume average particle diameter thereof is 7.5 .mu.m. By using a
toner of a spherical shape or a shape similar to a sphere, it
becomes possible to greatly decrease the amount of untransferred
toner, as compared with a toner of an indefinite shape manufactured
by crushing classification, and this toner is very advantageous in
the cleanerless method.
Also, the carrier used in the present embodiment is produced by the
polymerizing method, and is a resin magnetic carrier comprising a
core made of resin in which a magnetic material or a nonmagnetic
metal oxide is dispersed and covered with silicone resin. The
volume average particle diameter of the carrier is 40 .mu.m, and
the resistance value thereof is 1.0.times.10.sup.12 .OMEGA.cm.
The resistance of the carrier was measured by the use of a
measuring apparatus shown in FIG. 2. An upper electrode 34 and a
lower electrode 35 are disposed in a cell E so as to contact with
the carrier 33, and a predetermined voltage is applied from a
voltage source 36 to between these electrodes. An electric current
flowing at that time was measured to thereby find the specific
resistance of the carrier. The specific resistance measuring
conditions used in the present embodiment are the contact area
between the filling carrier and the electrodes in about 2.3
cm.sup.2, the thickness (d) is about 2 mm, the load of the upper
electrode 34 is 180 g, and the intensity of a measuring electric
field is 3.times.10.sup.5 V/m; and the measuring environment is:
atmospheric temperature is 23.degree. C., and relative humidity is
50%.
Regarding the representation of the resistance value, in the
present embodiment, the common logarithm of the resistance value R
is X, and R is represented as R=1.0.times.10.sup.x. For example, a
numerical value of 5.0.times.10.sup.11 (mantissa 5.0, exponent 11)
is about 11.7 when the common logarithm thereof is taken and
therefore, when the resistance value R is 5.0.times.10.sup.11
.OMEGA.cm, this is represented as 1.times.10.sup.11.7
.OMEGA.cm.
The toner is minus-charged by the triboelectrification of the toner
and the carrier.
The weight percentage (toner density) of the toner in the
two-component developer is adjusted to 8% in the initial state of
the installation of the developing device 1 according to the
present embodiment.
The toner in the two-component developer is consumed by the
development of the electrostatic latent image and the toner density
is decreased. An amount of toner corresponding to the decrease is
suitably supplied from a toner supplying tank (not shown) to the
two-component developer in the developer container 2 in accordance
with the amount of consumed toner.
The determination of the amount of toner supply from the toner
supplying tank to the developer container 2 is effected by a method
of measuring the quality of reflected light for a reference toner
image formed by the reference latent image on the photosensitive
drum 28 being developed, by an optical sensor 31. The amount of
adhering toner on the reference toner image changes in accordance
with the toner density of the two-component developer and
therefore, how much toner consumption was done for the developing
device 1, that is, an amount of toner supply required, can be
determined.
The toner supplied to the developer container 2 is agitated and
mixed with the two-component developer by an agitating member 6 and
is subjected to triboelectrification, whereby it is given a
predetermined amount of charge.
The toner image formed on the photosensitive drum 28 in the manner
described above is electrostatically transferred to the surface of
transfer paper P by the transfer charger 23. The transfer paper P
to which the toner image has been transferred is heated and
pressurized by the fixing device 25, whereby the toner image is
fixed thereon. The toner image is thus permanently fixed.
An untransferred toner residual on the photosensitive drum 28 after
the transfer is collected into the developing device 1 by a
cleanerless method and is recycled. The collection is done in the
following manner.
The toner having adhered from the developing device 1 to the
surface of the photosensitive drum 28 during the development is
charged to minus. This toner is transferred to the transfer paper P
while receiving a bias of the plus polarity applied to the transfer
charger 23 during the transfer. At this time, although slightly,
particles which were plus in polarity in the toner and particles
which were charged at the transferring step by the transfer charger
23 and became plus in polarity remain on the surface of the
photosensitive drum 28, and these particles become the
untransferred toner. Charging in the minus direction by the
auxiliary charging brush 32 is first effected on this untransferred
toner having the plus polarity. A DC voltage of -700V as an
auxiliary charging bias is applied from a bias voltage source, not
shown, to the auxiliary charging brush 32 provided with an
electrically conductive brush. Since the auxiliary charging bias is
of the minus polarity, the auxiliary charging brush 32 catches the
untransferred toner of the plus polarity. The untransferred toner
temporarily stagnates between the photosensitive drum 28 and the
auxiliary charging brush 32. The untransferred toner is subjected
to the charging by the auxiliary charging bias and becomes minus in
polarity while it stagnates. The charge amount of the untransferred
toner subjected to the auxiliary charging bias has become great in
absolute value toward the minus side relative to the toner in the
developing device 1. The charge amount of the untransferred toner,
however, becomes on average small in absolute value by the AC
component of the primary charging bias. Therefore, the
untransferred toner having passed the primary charger 21 becomes
substantially equal in charge amount to the toner in the developing
device 1. The untransferred toner having had its charge amount
regulated in this manner is stripped from the photosensitive drum
28 in the first opposed portion 29A by the scraping-off force of
the two-component developer and the electric field by the AC
component of the developing bias, and is moved from the surface of
the photosensitive drum 28 to the first developing roller 3A by an
electric force by the potential difference of Vback. In this
manner, the untransferred toner is removed from the surface of the
photosensitive drum 28.
The characteristic portion of the developing device according to
the present embodiment will now be described in detail.
The feature of the developing device according to the present
embodiment is that the cleaning performance in the cleanerless
method can be maintained good even under a severe condition such as
a case where images great in amount of toner consumption are
continuous. That is, the developing device according to the present
embodiment has the feature that it can maintain the cleaning
performance good even if the amount of toner consumption is
great.
As previously described, the performance of the cleanerless method
depends greatly on the cleaning performance and fogging
characteristic by the developing rollers 3A and 3B. If either of
the cleaning performance and the fogging characteristic is bad, the
amount of the toner moving round together with and on the
photosensitive drum 28 (the toner resulting from the untransferred
toner) will increase.
The toner moving round together, if in a predetermined amount or
less, steadily exists on the photosensitive drum 28, but does not
hinder the image and the cleaning performance. However, if the
amount of the toner moving around together reaches such an amount
as hinders the charging by the primary charger 21, the difference
between Vdark and the DC component of each developing bias will be
reduced and both of the cleaning performance and the fogging
characteristic will be aggravated, and the amount of the toner
moving around together will be further increased.
Particularly, in a case where images great in amount of toner
consumption are continuously transferred to the transfer paper P,
the charges imparted to the toner by agitation may become deficient
due to an increase in the supplied toner. In such a case, the
vicious circle as described above may be caused by the aggravation
of the fogging characteristic.
That is, to stably materialize the cleanerless method, it becomes
necessary to suppress the amount of the toner moving around
together to a predetermined amount or less by improvements in the
cleaning performance and the fogging characteristic even under a
severe condition under which images great in amount of toner
consumption are continuously transferred to the transfer paper
P.
So, the developing device according to the present embodiment is
made into a construction wherein the two-component developer is
conveyed in a direction opposite to the direction of movement of
the photosensitive drum 28 and the toner is consumed without fail
by the second developing roller 3B, whereby particularly even when
the amount of toner consumption is great, design is made such that
the toner density of the two-component developer conveyed to the
first developing roller 3A is lowered. If such a construction is
adopted, in a case where the amount of toner consumption is great
which is a condition under which the amount of the toner moving
around together is liable to increase, the toner density of the
two-component developer on the first developing roller 3A is
automatically lowered in accordance with the degree thereof, and
the cleaning performance can be improved.
To lower the toner density of the two-component developer on the
first developing roller 3A, as described above, it is necessary to
cause the toner to adhere to the electrostatic latent image, on the
second developing roller 3B on the side which supplies the
two-component developer onto the first developing roller 3A. For
this purpose, it is necessary that the developing action by the
first developing roller 3A become less for the electrostatic latent
image and room for the toner adherence by the development by the
second developing roller 3B to be effected by left. This
construction will hereinafter be described with reference to FIG.
3.
FIG. 3 shows the relations among the latent image surface potential
Vdark and Vlight on the photosensitive drum 28, the DC component
Vdev of the developing bias, the toner surface layer potential Vt1
after the completion of the development on the first developing
roller 3A, and the toner surface layer potential Vt2 after the
completion of the development on the second developing roller
3B.
The latent image surface potential on the photosensitive drum 28
and the toner surface layer potential are measured by a surface
potential meter Model 344 produced by Trek Japan Corporation. The
latent image surface potential may be measured near the upper or
lower portion of the developing device 1, or may be measured with
the first developing roller 3A and the second developing roller 3B
experimentally removed and at the position thereof. The toner
surface layer potential Vt1 is difficult to measure by an actual
apparatus construction, but can experimentally be measured with the
second developing roller 3B removed and by the same amount of
two-component developer as in an actual operation being carried on
and conveyed by the first developing roller 3A, and a measuring
probe being opposed to the vicinity of the lower portion of the
second developing roller 3B or the developing device 1. Also, the
toner surface layer potential Vt2 can be measured by measuring the
surface of the photosensitive drum 28 near the lower portion of the
developing device 1.
In the first opposed portion 29A, the toner surface layer potential
Vt1 immediately after the development by the first developing
roller 3A has been completed is -235 V and has not yet reached -350
V which is the value of the DC component Vdev of the developing
bias applied to the first developing roller 3A. This is because due
to a high-resistance carrier being used, the impedance in the first
opposed portion 29A is great and an electrical transitory
phenomenon called the developing step is not completed ("the
completion of the developing step") within the range of the first
opposed portion 29A. Therefore, there is a potential difference of
115 V between the surface layer potential Vt1 of the toner image
formed in the first opposed portion 29A and the DC component Vdev
of the developing bias applied to the second developing roller 3B,
and there remains sufficient room for effecting toner consumption
on the second developing roller 3B. When for this potential
difference, development is effected by the second developing roller
3B, the toner surface layer potential Vt2 becomes -300 V, that is,
an amount of toner corresponding to 65 V in terms of the toner
surface potential has adhered onto the photosensitive drum 28, and
this shows that the toner consumption by the second developing
roller 3B has been sufficiently effected.
The two-component developer having had its toner consumed by the
second developing roller 3B in this manner is conveyed to the first
developing roller 3A, and development and the cleaning step are
effected in the first opposed portion 29A.
Regarding the cleaning performance, the untransferred toner
collecting capability of the two-component developer is enhanced by
the toner density being lowered. This is because by the toner
density being lowered, an area which can receive the untransferred
toner is increased on the carrier surface in the two-component
developer.
This action becomes remarkable particularly when an image great in
amount of toner consumption is formed and therefore, even under a
severe condition such as a case where images great in amount of
toner consumption are continuous, the cleaning performance in the
cleanerless method can be maintained good.
The effect of the high-resistance carrier in the present embodiment
will be described here in detail.
The "completion of the developing step" described above can be
expressed by the use of an index called "latent image charging
rate" as will be described below. The latent image charging rate is
represented by the ratio of the potential difference (Vlight-Vt)
formed by the developed toner image to the latent image contrast
potential (Vlight-Vdev). That is, the latent image charging rate
can be represented as the latent image charging
rate=(Vlight-Vt)/(Vlight-Vdev).
The "completion of the developing step" shows that this latent
image charging rate converges to a predetermined value (Vdev) in
accordance with an exponential function, and substantially becomes
100%. That is, the fact that the developing step is not complete in
a system using the high-resistance carrier like the present
embodiment shows that the above-mentioned latent image charging
rate has not yet reached 100%.
The time constant of the above-mentioned exponential function
depends on the impedance between the photosensitive drum 28 and the
developing roller 3A and between the photosensitive drum 28 and the
developing roller 3B. When this impedance is high, the time
constant becomes great, and much time is required until the
phenomenon is saturated. Conversely, when the impedance is low, the
time constant becomes small, and the time until the phenomenon is
saturated becomes short.
FIG. 4 shows as a comparative example the relations among the
latent image surface potential on the photosensitive drum 28, the
DC component Vdev of the developing bias and the toner surface
layer potential Vt1 and Vt2 after the completion of the development
on the first developing roller 3A and the second developing roller
3B in a case where the resistance value as the carrier is
1.0.times.10.sup.7 .OMEGA.cm.
It is when for example, an electrically conductive substance is
much contained in the core of a resin carrier or when ferrite is
used as the core of the carrier that the resistance of the carrier
becomes 1.0.times.10.sup.7 .OMEGA.cm.
In the first opposed portion 29A, the toner surface layer potential
Vt1 immediately after the development by the first developing
roller 3A becomes -350 V, which is substantially the same as -350 V
which is the value of the DC component Vdev of the developing bias
applied to the first developing roller 3A. This is because as
compared with the high-resistance carrier as in the present
embodiment, the impedance in the first opposed portion 29A is
small, and there is an electrical transitory phenomenon called the
developing step, but the time constant is small for the system of
the present embodiment and therefore, the phenomenon is
substantially instantaneously saturated. That is, this means that
when use is made of a carrier having a resistance value of
1.0.times.10.sup.7 .OMEGA.cm, the latent image charging rate
reaches 100% by the development by the first developing roller
3A.
Therefore, a potential difference cannot be formed between the
surface layer potential Vt1 of the toner image formed in the first
opposed portion 29A and the DC component Vdev of the developing
bias applied to the second developing roller 3B, and there is no
room for effecting toner consumption on the second developing
roller 3B. Therefore, when the resistance value of the carrier is
10.sup.7 .OMEGA.cm, an effect like that of the present embodiment
cannot be obtained.
In order that as in the present embodiment, the toner may be
consumed without fail by the second developing roller 3B, it is
necessary to use a carrier having a resistance value of
1.0.times.10.sup.9 .OMEGA.cm or greater.
However, if the resistance value of the carrier exceeds
1.0.times.10.sup.13 .OMEGA.cm, the aggravation of the developing
characteristic will become remarkable and it will become difficult
to obtain desired image density although an effect like that of the
present embodiment will be obtained. Also, collaterally with the
aggravation of the developing characteristic, a phenomenon called
"edge enhancement" meaning that the toner adheres excessively to
the edge portion of the latent image and the density thereof
becomes high is aggravated, and this is not preferable.
Therefore, it is necessary that the resistance value of the carrier
used in the present embodiment be within a range of
1.0.times.10.sup.9 .OMEGA.cm to 1.0.times.10.sup.13 .OMEGA.cm.
That is, when as previously described, the common logarithm of the
resistance value R of the carrier is defined as X, it is necessary
that 9.0.ltoreq.X.ltoreq.13.0 (1) be materialized.
A description will be further made of various conditions for
suitably obtaining the effect of the present invention.
FIG. 5 shows the relative relation of an AC electric field applied
to between the first developing roller 3A and the photosensitive
drum 28 and between the second developing roller 3B and the
photosensitive drum 28 which shows the feature of the present
embodiment. The values of these electric fields are indicated on
the axis of abscissas and the axis of ordinates.
A first necessary condition for suitably obtaining the action of
the present embodiment is that when the closest distance between
the first developing roller 3A and the photosensitive drum 28 is
defined as SD1 (m), and the closest distance between the second
developing roller 3B and the photosensitive drum 28 is defined as
SD2 (m), and the peak-to-peak voltages of the AC components of the
bias voltages applied to the first developing roller 3A and the
second developing roller 3B are defined as V1 (V) and V2 (V),
respectively, and the volume resistivity of the aforementioned
magnetic carrier is 1.0.times.10.sup.x (.OMEGA.cm),
V1/SD1-V2/SD2.ltoreq.(0.65x-4.5).times.10.sup.6(V/m) (2) is
satisfied.
Regarding expression (2), if the value of V1/SD1-V2/SD2 exceeds
(0.65x-4.5).times.10.sup.6 (V/m) (an area rightwardly below a
straight line 2 indicative of expression (2) in FIG. 5), the
developing capability of the first developing roller 3A for the
electrostatic latent image will become high and the developing
capability of the second developing roller 3B for the electrostatic
latent image will be decreased, and this leads to the undesirable
possibility that the toner consuming effect by the second
developing roller 3B may be hindered and it becomes impossible to
sufficiently obtain the effect of the present invention.
As shown in expression (2), this relative relation is varied by the
resistance value of the carrier, and the greater is the value of X,
the more greatly can be set V1/SD1 and the smaller can be set
V2/SD2. That is, the greater is the resistance value of the
carrier, the more easily can be obtained the toner consuming effect
by the second developing roller which is the characteristic effect
of the present invention.
Next, a second necessary condition for suitably obtaining the
action of the present embodiment is that each of V1/SD1 and V2/SD2
is within an optimum range. The condition is as follows:
1.2.times.10.sup.6.ltoreq.V1/SD1.ltoreq.(0.35x+3.5).times.10.sup.6
(V/m) (3)
(0.3x-1.5).times.10.sup.6.ltoreq.V2/SD2.ltoreq.9.0.times.10.sup.6
(V/m) (4)
Expression (3) relates to the AC electric field applied to the
first developing roller 3A.
If V1/SD1 is below 1.2.times.10.sup.6 (V/m) (an area on the left
side of a straight line 2-1 indicative of the left side of
expression (3) in FIG. 5), the toner stripping capability for the
untransferred toner by the AC electric field will not be obtained
in the first opposed portion 29A between the first developing
roller 3A and the photosensitive drum 28, and this leads to the
undesirable possibility that the cleaning performance on the first
developing roller 3A may not be sufficiently obtained.
Also, if V1/SD1 exceeds (0.35x+3.5).times.10.sup.6 (V/m)(an area on
the right side of a straight line 2-2 indicative of the right side
of expression (3) in FIG. 5), the developing capability of the
first developing roller 3A for the electrostatic latent image will
be enhanced and the toner consuming effect by the second developing
roller 3B will be hindered, and this leads to the undesirable
possibility that the effect of the present invention cannot be
sufficiently obtained.
As shown in the right side of expression (3), the upper limit value
of V1/SD1 is varied by the resistance value of the carrier. When
the resistance value of the carrier is high, the degree to which
the developing capability of the first developing roller 3A for the
electrostatic latent image is enhanced is small and therefore, it
is relatively difficult for the toner consuming effect by the
second developing roller 3B to be hindered. Therefore, it is
possible to set the AC electric field applied to the first
developing roller 3A to a great level to thereby enhance the
cleaning effect.
Expression (4) relates to the AC electric field applied to the
second developing roller 3B.
If V2/SD2 is below (0.30x-1.5).times.10.sup.6 (V/m) (an area below
a straight line 3-1 indicative of the left side of expression (4)
in FIG. 5), the developing capability of the second developing
roller 3B for the electrostatic latent image will be decreased and
the toner consuming effect by the second developing roller 3B will
be hindered, and this leads to the undesirable possibility that the
effect of the present invention cannot be sufficiently
obtained.
As shown in the left side of expression (4), the lower limit value
of V2/SD2 is varied by the resistance value of the carrier. When
the resistance value of the carrier is high, the developing
capability of the second developing roller 38 for the electrostatic
latent image is liable to be decreased and the toner consuming
effect by the second developing roller 3B becomes liable to be
hindered and therefore, it is necessary to set the AC electric
field applied to the second developing roller 3B to a relatively
great level.
Also, if V2/SD2 exceeds 9.0.times.10.sup.6 (V/m) (an area above a
straight line 3-2 indicative of the right side of expression (4) in
FIG. 5), insulation destruction may occur in the opposed portion
between the first developing roller 3A and the photosensitive drum
28 to thereby spoil the image forming function, and this is not
preferable.
Further, to suitably obtain the effect of the present invention, it
is preferable that design be made such that when the values of the
DC components of the bias voltages applied to the first developing
roller 3A and the second developing roller 3B are made the same,
and when the surface potential of the maximum density portion of
the electrostatic latent image is defined as Vlight (V), and the
toner surface layer potential on the latent image bearing member
after the maximum density portion of the electrostatic latent image
has been developed by the first developing roller 3A is defined as
Vt1 (V), and the toner surface layer potential on the latent image
bearing member after the aforementioned maximum density portion has
been developed by the second developing roller 3B is defined as Vt2
(V), 0.15.ltoreq.(Vt2-Vt1)/(Vt2-Vlight).ltoreq.0.50 (5) is
satisfied.
In expression (5), (Vt2-Vlight) represents the surface potential
difference by the toner used to develop by the entire developing
device according to the entire developing device according to the
present embodiment, and (Vt2-Vt1) represents the surface potential
difference by the toner used to develop by the second developing
roller 3B. In the multi-stage developing device like the present
embodiment using two-component contact AC development, the charge
amount per weight of the toner used to develop on each developing
roller may be considered to be substantially constant and
therefore, this value indicates the percentage of the amount used
to develop by the second developing roller 3B, of the toner used to
develop.
The aforedescribed construction of the present embodiment is a
construction in which the value of (Vt2-Vt1)/(Vt2-Vlight) is
substantially 0.15 or greater, and when this is below 0.15, there
is the undesirable possibility that the toner consuming effect by
the second developing roller 3B becomes deficient and the effect of
the present invention cannot be sufficiently obtained.
Also, within the range of the aforedescribed construction of the
present embodiment, it is considered that (Vt2-Vt1)/(Vt2-Vlight)
does not substantially exceed 0.50. Supposing a case where this
value exceeds 0.50, it shows a condition under which the toner
consuming effect by the second developing roller 3B is sufficiently
obtained, but conversely the developing property by the first
developing roller 3A is too low. As previously described, in the
construction of the present embodiment, such a condition under
which the developing performance of the first developing roller 3A
is low tends to decrease the cleaning performance of the first
developing roller 3A at the same time, and therefore is not
preferable.
The method of measuring Vlight (V), Vt1 (V) and Vt2 (V) has already
been described.
Also, in the present embodiment, it is preferable that design be
made such that the aforementioned SD1, SD2, V1 and V2 are as
follows: 0.2.times.10.sup.-3
(m).ltoreq.SD1.ltoreq.1.0.times.10.sup.-3 (m) (6)
0.2.times.10.sup.-3 (m).ltoreq.SD2.ltoreq.1.0.times.10.sup.-3 (m)
(7) 500 (V).ltoreq.V1.ltoreq.3000 (V) (8) 500
(V).ltoreq.V2.ltoreq.3000 (V) (9)
Regarding the expressions (6) and (7), if SD1 and SD2 are below
0.2.times.10.sup.-3 (m), the possibility of the stagnation of the
developer in the opposed portions between the developing rollers
and the photosensitive drum occurring will become great, and the
possibility that the electric field in the opposed portions by the
bias voltage strengthens to thereby cause insulation destruction
will become great, and this is not preferable. Also, if SD1 and SD2
exceed 1.0.times.10.sup.-3 (m), the possibility that the contact of
the developing rollers with the photosensitive drum 28 weakens to
thereby decrease the cleaning performance and the developing
performance will become great, and this is not preferable.
Regarding the expressions (8) and (9), if V1 and V2 are below 500
(V), the possibility of the reductions in the cleaning performance
and the developing performance due to the weakening of the AC
electric field in the opposed portion between the developing roller
and the photosensitive drum 28 will become great, and this is not
preferable. Also, if V1 and V2 exceed 3000 (V), the possibility of
the insulation destruction due to the strengthening of the electric
field in the opposed portion caused by the bias voltage will become
great, and this is not preferable.
Second Embodiment
Developing Device According to the Second Embodiment
The developing device according to this embodiment is one in which
the AC bias waveform in the developing device according to the
first embodiment has been changed. The AC component of a developing
bias in the present embodiment, as shown in FIG. 6, in a blank
pulse waveform of a construction which repeats the action of
generating a rectangular wave having a frequency of 8 kHz and a
peak-to-peak voltage of 1.8 kV by an amount corresponding to two
cycles, and thereafter discontinuing it by an amount corresponding
to two cycles.
In the present embodiment, design is made such that a constant
voltage and the DC component of the developing bias when being
discontinued by an amount corresponding to two cycles in FIG. 6
coincide with each other. This is because the AC component of the
developing bias in the present embodiment is symmetrical with
respect to the constant voltage when the AC component is being
discontinued by an amount corresponding to two cycles.
In the present invention, however, the AC waveform of the
developing bias is not always restricted to a vertically
symmetrical one, and particularly in a case where the AC waveform
is not vertically symmetrical in a waveform provided with a
discontinued portion as in the present embodiment, a voltage value
set as the discontinued portion and the value of the DC component
of the developing bias do not always coincide with each other.
As previously described, the potential difference between the
potential Vdark of the electrostatic latent image and the
developing bias Vdev is called Vback (fogging ensuring potential),
and in the present embodiment, it is 150V. When this Vback has
become small relative to a center set value, fogging gradually
increases in accordance therewith. Here, when the blank pulse
waveform in the present embodiment is used as the developing bias,
the tendency of fogging increasing from its minimum state can be
made small, as compared with a case where use is made of a
rectangular wave like that in the first embodiment. That is, even
when Vback has become small due to faulty charging by the primary
charger 21 resulting from the faulty collection of the
untransferred toner, the degree of aggravation of fogging is small
as compared with the first embodiment. Therefore, the effect of the
present invention can be obtained more effectively, and this is
preferable.
While the embodiments of the present invention have been described
above, the detailed construction of the present invention is not
always restricted to these embodiments, but can assume various
forms within a scope in which the objects of the present invention
can be suitably attained. Of course, it is also possible to
suitably combine and use the methods of the above-described
embodiments, and in view of various conditions, the condition can
be adjusted so that the effect of the present invention may be
appropriately obtained.
For example, the magnetic pole arrangement of the two-component
developer delivering and receiving portion between the developing
rollers as described above is not restricted to a different pole
arrangement, but the same poles may be arranged.
For example, regarding the method of determining the amount of
toner supply, as there have heretofore been made various
propositions, there are a method of detecting toner density by an
optical sensor or a permeability sensor in the developer container
2, a method of detecting the amount of toner consumption by image
information or the light emission frequency of the laser beam
scanner 22, etc. Use may be made of any method including the
methods mentioned in the embodiments, or a combination of those
methods. Any method adopted is similar in that when an image of
high image density or a high area rate is taken, the amount of
toner supply increases in accordance therewith.
Also, regarding the transfer of the toner image from the
photosensitive drum 28, it is known that the use of an intermediate
transfer belt of which the physical properties can be designed
under a predetermined condition is more stable and can more
decrease the amount of untransferred toner than to transfer paper
having various physical property values, and therefore this method
may also be used.
Further, it is also possible to make the developing biases applied
to the first developing roller 3A and the second developing roller
3B discrete from each other, and set the fogging and the amount of
toner consumption by the first developing roller 3A so that the
effect of the present invention may be easily provided. In this
case, however, the necessity of individually disposing the
developing bias voltage sources occurs and the potential difference
(particularly the AC component) between the first developing roller
3A and the second developing roller 3B occurs and therefore, it
becomes necessary to separately provide a countermeasure for the
leak between the first developing roller 3A and the second
developing roller 3B, and this results in the complication of the
construction and is therefore not preferable.
The copying machine 50 according to the present embodiment is
provided with the developing device 1 which may be great in the
amount of toner construction but is not reduced in the cleaning
performance, and can therefore form a good quality of image on the
transfer paper.
The developing apparatus according to an embodiment of the present
invention is designed such that even though the amount of toner
consumption may be great, lastly the toner is consumed on the
second developing member opposed to the electrostatic latent image
on the image bearing member and therefore, the density of the toner
first conveyed to the first developing member opposed to the
electrostatic latent image on the image bearing member is
automatically lowered, and the cleaning performance is hardly
reduced.
The image forming apparatus according to an embodiment of the
present invention is provided with the developing apparatus which
may be great in the amount of toner consumption but is not reduced
in the cleaning performance, and can therefore form a good quality
of image on a sheet.
While the invention has been described with reference to the
structure disclosed herein, it is not confined to the details set
forth and this application is intended to cover such modifications
or changes as may come within the purpose of the improvements or
the scope of the following claims.
This application claims priority from Japanese Patent Application
No. 2004-201171 filed on Jul. 7, 2004, which is hereby incorporated
by reference herein.
* * * * *